Injection molding is carried out in molding presses that serve to hold the components forming the cavity mold in secured, sealed relation, to receive hot, molten plastic injected at pressures in the order of twenty thousand pounds per square inch. The molten plastic is injected through an injector nozzle assembly that abuts the mold cavity at the gate of the mold cavity. Each injector has a replaceable nozzle tip that cooperates thermally with the gate, and through which the plastic is forced.
The requirement for successful molding is that the gate land, that zone of the mold surrounding the gate, be kept cold, by circulating cooling water, while the nozzle tip is maintained above the solidification temperature of the plastic, by the passage of the hot plastic therepast, and by the operation of electric nozzle heaters.
At the end of each injection cycle, the injected plastic solidifies in the mold, including the plastic in the gate, in the form of a slug. Upon opening of the mold the molded article breaks away from the gate and the slug, upon ejection, leaving the severed slug attached to the gate land, in blocking relation with the gate.
When the mold recloses for the next cycle, under design conditions the injection pressure is sufficient to displace the slug into the mold, where it remelts into the plastic stream. However, in the event that the nozzle tip temperature is lowered below a desired minimum level, the degree of plastic solidification can extend so as to freeze the plastic in the gate zone to such an extent that the slug builds up around the gate land and can no longer be displaced into the mold cavity during the succeeding injection cycle, so that defective injection occurs, or injection is totally blocked.
Thus it will be seen that thermal conditions at the nozzle can be critical, and require to be maintained as close to ideal as possible.
In the event that the nozzle tip partially unscrews during molding, it will move towards the gate. This results in:
1. Disturbance of thermal conditions in the mold; PA1 2. Partial or complete blocking of the gate, usually by extended "freezing" of plastic before the gate. PA1 1. Building-up of the male thread profile of the nozzle tip by a plating or coating such as chrome or nickel plating; PA1 2. The insertion of a locking set-screw against the side of the nozzle tip; PA1 3. Applying two different thread forms to the nozzle tip, to ensure jamming of the threads; PA1 4. Using a different type of material, such as H13 tool steel instead of the more usual copper-beryllium alloy; but this can lead to seizing of the nozzle tip threads to the nozzle housing, or freezing of plastic in the gate due to lower thermal conductivity. PA1 an elongated nozzle housing having an internally threaded hollow barrel for passage of the plastic; PA1 a hollow injector nozzle tip, having a threaded body portion located within the barrel for securing the nozzle tip in coaxial threaded relation in the barrel, a portion of the nozzle tip within the barrel having a radial clearance from the barrel, the nozzle tip extending outwardly beyond the nozzle housing; PA1 composite keyway means having at least one first keyway recess portion in the barrel located in inward facing relation with the nozzle tip; PA1 at least one second keyway recess portion in the nozzle tip in facing relation with the at least one first recess portion, the first and second recess portions when in substantial aligned relation forming a composite keyway to receive a key member in close fitting inserted relation therein, to immobilize the nozzle tip in relation to the nozzle housing.
These conditions lead to "short shots", ie. partial filling of the mold and a totally defective product.
In one arrangement of injector nozzles the nozzle housing is seated in sealing relation within a nozzle cavity adjacent the gate.
The injector nozzle has a centrally located, axially protruding nozzle tip through which the injected plastic passes into the adjoining gate orifice. In the event that the injector nozzle backs off from its seat, upon its securing threads, it moves further into the gate, thus restricting or even blocking-off the gate.
Such backing off of an injector nozzle tip is highly undesirable, and necessitates the termination of molding operation, including the shutting down of the press, the stripping down of the mold to access the nozzle tip, and includes the removal of a mass of solidified plastic, and any other required remedial action on the defective assembly.
The immobilization of this type of injector nozzle tip against such backing-off, in order to prevent molding failure has proved to be difficult to achieve. Due to the close space limitations imposed by the environment within the mold, coupled with the imperative need to maintain balanced nozzle tip thermal conditions and geometry, allied with the varying temperatures required for different melts, and the extremely high injection pressure of the hot plastic, an effective nozzle tip locking means has proved to be elusive, and the problem of nozzle back-off is recurrent.
In the case of a multi-cavity mold, where the flow symmetry and balanced flow conditions are usually of vital importance, the backing-off malfunction of one nozzle tip almost inevitably unbalances the flow of plastic to the remaining cavities, with consequent shorts and/or flashing due to "over packing" being produced by them also.
Included in any prior attempts to deal with this problem are:
One present practice is to torque home the nozzle tip in controlled jamming relation in the nozzle housing, with varying success.
Another approach, the provision of thermoset plastic spacers attached to the outer end of the nozzle tip, to bear against the land that defines the mold gate, is costly, and the spacers are prone to damage during routine set-up and maintenance.
Other mechanical design solutions which tend to disturb the necessary delicate thermal balance at the nozzle tip have proved to be unacceptable.